(i) Field of the Invention
The present invention relates to a structure of a semiconductor laser for use in an optical communications system. More specifically, it relates to a semiconductor laser which enables a low threshold, a high efficiency and a high output power even at a high temperature.
(ii) Description of the Prior Art
With the progress of an optical communications technique, the applicable fields of semiconductor lasers are rapidly spreading from a trunk transmission system to systems such as a subscriber system, an LAN and a data link. The semiconductor lasers which can be used in these fields have been used in large quantities in various environments, and there have been demands for semiconductor lasers with excellent environmental resistance and with lower prices. Hence, research and development have been actively conducted. In particular, there has been desired a laser having a low threshold, a high efficiency and a high output power even at a high temperature in order to make the device temperature control-free under no bias from the viewpoints of miniaturization and the cost reduction of a laser module.
In consequence, there have been developed a technique in which a strain MQW structure is employed for an active layer to change a band structure and to thereby increase a gain, and another technique in which a p-type InP substrate is used and a current block structure is formed thereon by a metal organic vapor phase epitaxial growth (MOVPE growth) to reduce a leak current. FIG. 8 shows a structure of a Planer Buried Heterostructure semiconductor laser diode (PBH-LD) for optical communications in which a strain MQW active layer 3 and a current block layer are formed on a p-type InP substrate 1 by the MOVPE growth (5th Optoelectronics Conference Technical Digest, 14D1-2, July 1994). In order to operate this semiconductor laser at a high temperature at a high output, it is necessary to increase the breakdown voltage of the current block layer, and hence, an n-type InP current block layer 7 has been doped at a high concentration, and a p-type InP buried layer 5 and a p-type InP current block layer 9 have been doped at a low concentration.
However, it has been revealed that, in the conventional structure shown in FIG. 8, an optical output tends to be saturated at a high temperature at a high output and the breakdown voltage is low.